P17.32.b Role of Interval Magnetic Resonance Imaging in Adaptive Intensity Modulated Proton Therapy in CNS Tumors

Abstract BACKGROUND Patients treated with fractionated radiotherapy for central nervous system (CNS) tumors may experience changes in target volumes (TV) and organs at risk (OARs). High-precision techniques like intensity-modulated proton therapy (IMPT) require adaptive radiation therapy (ART) to account for these changes and optimize outcomes. This study explores the role of interval imaging and ART in CNS tumor treatment and also evaluates the impact of ART, specifically in reducing toxicity and improving tumor control by adapting radiation plans based on interval MRI findings during the treatment course. MATERIAL AND METHODS This retrospective study included patients treated with IMPT for CNS tumors between August 2023 and March 2025. Interval Magnetic resonance imaging (MRI) was scheduled based on tumor characteristics, such as cystic tumors, presence of cavities, subdural collections, and skull base tumors to monitor anatomical changes during treatment. For patients undergoing ART, re-contouring and re-planning of target volumes and organs at risk were systematically performed whenever a significant change in tumor volume or anatomical structure was identified, ensuring optimal dose delivery and minimizing exposure to surrounding healthy tissues. RESULTS Among 116 patients, 92 interval magnetic resonance imaging (MRI) scans were performed in 62 patients (53%). Adaptive radiotherapy (ART) was implemented following 53 of these 92 scans (58%), ultimately affecting 29 patients (25% of the cohort). Changes in target volume (TV) were detected in 53 instances, while modifications involving adjacent organs at risk (OARs) were identified in 5 cases. Of the 14 patients undergoing re-irradiation, 3 exhibited significant alterations in TV. The most common tumor types requiring ART included craniopharyngioma (n = 8), diffuse glioma (n = 8), ependymoma (n = 5), and skull base tumors (n = 2), with craniopharyngioma being the most frequently adapted diagnosis. ART was predominantly applied in benign and low-grade tumors characterized by small clinical target volume (CTV) margins and in patients undergoing re-irradiation. Notably, over 80% of interval MRIs were performed during the third and fourth weeks of treatment. Fourteen patients (out of the 62 who underwent MRI) required repeat computed tomography (CT) simulations for adaptive re-planning due to observed changes in tissue composition or alterations in beam path geometry. CONCLUSION Enhancing the efficiency of adaptive radiotherapy (RT) through technological advancements such as MR-guided imaging, improved auto-segmentation tools, and data-driven targeting strategies holds significant promise for improving clinical outcomes. These innovations support more precise treatment delivery, reduced toxicity, and resource optimization, making adaptive RT more feasible for widespread clinical adoption.